MAGNETIC ANOMALY MAPPING AND SUBSURFACE MODELING OF ONONDAGA LAKE, NEW YORK

Onondage Lake, Syracuse, NY, is a relic of a system of proglacial lakes that formed during the glacial retreat about 14000 years ago. The lake is similar in morphology to the Finger Lakes, which are formed in the joint sets of central NY. Onondaga Lake has serious environmental issues that are related to industrial activity over the past century. A high-resolution magnetic survey was conducted on the lake to produce a magnetic anomaly map followed by subsurface geologic models. The field survey used a magnetic gradiometer with built-in GPS, and an inflatable motor boat. Previous testing showed that the motor boat has no effect on magnetic readings. Onondaga Lake covers ~18 sq km, and generally has an elongate shape. The survey involved short back and forth sweeps across the width, in addition to three cross lines along the length of the lake. At an average survey speed of 30 km/hr, the data points were collected (5 per sec) at an average spacing of 1.7 m. The total survey time was ~2.5 hours. Survey data was cleaned to eliminate incomplete reading due to instrument error and extreme magnetic anomalies (possible man-made objects on the lake bed). Data correction for diurnal variation was completed with repeat readings at a common point. The data was also analyzed at cross points to look for major differences. Of the ~33000 collected data points, ~31500 were used in the modeling. A magnetic anomaly map was produced using a smoothed, minimum curvature protocol. The anomaly maps showed that the lake resides over two magnetic gradients. One gradient runs parallel to the long axis of the lake, with the higher magnetic values at the NNW end of the lake and gradually lower toward the SSE direction. The second gradient runs perpendicular to the trend of the lake. This gradient trends WSW to ENE with the higher magnetic values on the western side of the lake. There is also a narrow magnetic lineament that extends form the northern end of the lake to the middle. The following conclusions have been reached thus far. The general N-S gradient may be explained by rock formations containing substantial evaporites under the southern end of the lake. The narrow magnetic lineament may be an ancient buried channel or river valley. This magnetic lineament merges with a wider anomaly in the vicinity of sand deposits that form the white bluffs on the west side of Onondaga Lake.